Porous membranes are used to filter various liquids and gases and remove contaminants such as particles, tissue debris, cells, microorganisms, bubbles, gel particles and the like from these fluids. These porous membranes can be formed from a polymeric composition, metals, ceramics and/or biological composition and can have a controlled and measurable porosity, a pore size and pore size distribution, and specified thickness. The porous membranes can be used alone or may be incorporated into a filter device such as a cartridge which can be inserted within a fluid stream to effect removal of particles and the like from the fluid.
Porous membranes are chemically resistant to the fluid being filtered and maintain the membrane's strength, porosity, chemical integrity, and cleanliness during filtration. For example, in the manufacture of microelectronic circuits, filters made from polymeric porous membranes are used to purify various corrosive or chemically active process fluids (liquids, supercritical fluids, and gases) to prevent particulate contaminants from causing defects and circuit failures. Fluid filtration or purification is usually carried out by passing the process fluid through the porous membrane from a zone of higher pressure on the upstream side of the porous membrane to a zone of lower pressure on the downstream side of the porous membrane. Thus, liquids, gases, and even supercritical fluids being filtered in this fashion experience a pressure drop across the porous membrane filter.
For liquids, this pressure drop or differential pressure across the porous membrane can result in the liquid on the upstream side of the porous membrane having a higher level of dissolved gases than the liquid on the downstream side of the porous membrane. The change in gas solubility and or gas compressibility in the liquid occurs because gases, such as air, have greater solubility in liquids at higher pressure compared with liquids at lower pressure. As the liquid passes from the upstream side of the membrane filter to the downstream side, dissolved gases can form bubbles, nucleate on particles or surface defects, or come out of solution in the membrane resulting in outgassing of the liquid. Liquids that can outgas, outgassing liquids, are commonly used in the manufacture of semiconductors and microelectronic devices and can include for example water, hydrogen peroxide, SC1 and SC2 cleaning baths, ozonated water, organic solvents such as alcohols, photoresists and antireflective coatings, developers, other aqueous acids and bases which can optionally contain an oxidizer, and salt containing solutions such as a buffered oxide etch (BOE).
A hydrophobic porous membrane is not directly wet with water and has a contact angle greater than 90 degrees for a water drop on the porous membrane. Filtration of outgassing liquids with a hydrophobic porous membrane can result in the dissolved gases coming out of the liquid at sites and surfaces on the hydrophobic membrane including the interior pore surfaces and the exterior or geometric surfaces. The hydrophobic porous membrane has greater affinity for the gas than the liquid. The gas that comes out of the liquid can accumulate and form gas pockets which adhere to the hydrophobic porous membrane surfaces and pores. As these gas pockets grow in size due to continued liquid outgassing, they begin to displace liquid from the pores of the hydrophobic porous membrane, ultimately reducing the effective filtration area of the hydrophobic porous membrane. This phenomenon is usually referred to as dewetting of the hydrophobic porous membrane since the fluid-wetted, or fluid-filled portions of the hydrophobic porous membrane are gradually converted into fluid-nonwetted, or gas-filled portions. Where dewetting occurs in a hydrophobic porous membrane, filtration in this portion of the membrane ceases with the result being a reduction of the overall filtration efficiency of the filter.
Chemically inert filter materials like Teflon® can be used to prevent membrane degradation in corrosive and chemically active fluids. Filter membranes that are incompatible with such fluids can undergo degradation which can lead to the chemical breakdown of the membrane composition. Membrane degradation may result in extractable materials being released from the filter during use, thus compromising the purity, integrity and cleanliness of the fluid being filtered. Membrane filters made from fluorine-containing polymers such as PTFE (polytetrafluoroethylene), or PFA (polytetrafluoroethylene-co-perfluoroalkoxy vinyl ether) can be utilized in these applications. Fluorine-containing polymers are well known for their chemical inertness and excellent resistance to chemical attack. One disadvantage of fluorine-containing polymers is that they are hydrophobic and therefore porous membranes made from such polymers are difficult to wet with aqueous liquids or other fluids which have surface tensions greater than the surface energy of the membrane. In order to wet the surface of a hydrophobic membrane with water or an aqueous fluid, it is current practice to first wet the membrane surfaces with low surface tension organic solvents such as isopropyl alcohol, followed by contact of the porous membrane surface with a mixture of water and an organic solvent which is then followed by contact of this exchanged membrane with water or an aqueous fluid. This process can create large volumes of solvent waste that must be disposed of and can consume large amounts of water due to the additional flushing of the filter cartridge with water. Alternatively, hydrophobic membranes can be wet with water under pressure. This pressure intrusion process is time consuming, expensive and ineffective for tight pore membranes, and can results in the rupture of thin porous membranes. Moreover, this process does not ensure that a substantial portion of the pores in the membrane are completely intruded with water.
In contrast to hydrophobic porous membranes, hydrophilic porous membranes are spontaneously wet upon contact with an aqueous liquid so that a treatment process for wetting the membrane surfaces is not used. That is, no prior treatment with an organic solvent or pressure intrusion, or mechanical energy such as by stirring is used in order for the hydrophilic membrane surface to be wet with water.
Moya, in U.S. Pat. No. 6,354,443 which is incorporated herein by reference in its entirety, discloses the modification and characterization of a porous membrane such as a polyperfluorocarbon membrane modified with a bound perfluorocarbon copolymer composition to render the entire surface non-dewetting. A porous membrane substrate or support is contacted with a perfluorocarbon copolymer composition in a solvent or diluent. Excess perfluorocarbon copolymer composition is removed from the surface with a solvent or diluent for the copolymer. The solvent or diluent does not remove the perfluorocarbon copolymer composition bound to the membrane surface. The membrane having the copolymer composition bound to its surface is then heat treated to improve the bond between the membrane substrate and the surface modifying perfluorocarbon copolymer composition. The perfluorocarbon copolymer composition is utilized in concentrations and amounts so that the membrane surface is completely modified while avoiding substantial blocking or plugging of the membrane pores. The surface modified porous membranes have a pressure drop that does not exceed an increase of greater than 25% as compared to the pressure drop across the unmodified membrane. Complete surface modification can be determined by staining with Methylene Blue dye.
Steuck in U.S. Pat. No. 4,618,533 discloses a composite porous membrane formed from a porous polymeric membrane on which is directly coated a cross-linked polymer that is not fluorinated. The composite porous membrane retains the porosity of the porous polymeric membrane. The composite porous membrane is formed from a porous polyvinylidene fluoride membrane which is directly coated with a polymer formed of a monomer and cross-linked with hydroxyalkyl acrylate.
Moya in U.S. Pat. No. 5,928,792 discloses a process for producing a porous membrane product having its surface completely modified with a perfluorocarbon copolymer composition. The porous membrane substrate is contacted with a solution containing a perfluorocarbon copolymer composition to bind the composition onto the substrate surface. The substrate is subjected to a mechanical force to remove excess perfluorocarbon copolymer composition and then is heat treated.
Moya in U.S. Pat. No. 6,179,132 discloses a porous membrane which is formed from a porous polyperfluorocarbon membrane substrate having its surface modified with a perfluorocarbon polymer composition. The modified surface is directly wet with water.
Moya in U.S. Pat. No. 7,094,469 discloses a porous or non-porous membrane or article formed from a fluorine-containing polymer substrate having its surface modified with an immobilized, such as by crosslinking and/or grafting, fluorocarbon having hydrophilic functional groups to provide a surface with improved hydrophilic characteristics as compared to the unmodified substrate. The modified surface is non-dewetting after being wet with an aqueous fluid or is directly wetted by an aqueous fluid. The fluorine-containing polymer substrate can be a porous membrane or can be a non-porous article. The immobilized fluorocarbon is formed from a monomer having formula: [T-SO2Y—SO2T′]−M+ in which T and T′ are identical or different and comprise an organic radical bearing at least one active polymerization function such as an unsaturation or a ring that can be opened; −M+ comprises an inorganic cation.
Moya in U.S. Pat. No. 7,112,363 discloses a porous or non-porous membrane or article formed from a fluorine-containing polymer substrate having its surface modified with a crosslinked or branched fluorocarbon polymeric composition having hydrophilic functional groups to provide a surface with improved hydrophilic characteristics as compared to the unmodified substrate. The fluorine-containing polymer substrate can be a porous membrane or can be a non-porous article. The surface comprising a crosslinked fluorocarbon, such as perfluorocarbon, polymeric composition having hydrophilic functional groups is provided having connecting bridges or crosslinks having sulfonyl or carbonyl-containing groups joining polymeric chains.
Moya in U.S. Pat. No. 7,288,600 discloses a crosslinked fluorocarbon polymeric composition having hydrophilic functional groups, crosslinked with fluorinated crosslinking groups, formed from a fluorocarbon polymer precursor, which is thermally and chemically stable and which can be rendered more hydrophilic than its fluorocarbon polymer precursor. The crosslinked perfluorocarbon polymeric composition, which is crosslinked with perfluorinated crosslinking groups are stable against degradation by virtue of contact with highly reactive reagents such as liquid compositions containing a base such as ammonium hydroxide, an oxidizer such as hydrogen peroxide or ozone and water, having a pH greater than about 9 such as special cleaning (SC) solutions, for example SC1 used during the manufacture of electronic components. According to U.S. Pat. No. 7,288,600, crosslinking moieties containing non-perfluorinated organic groups become degraded upon contact with these reagents and these non-perfluorinated chemical crosslinks are destroyed so that the crosslinked polymer loses its original degree of crosslinking. The crosslinked fluorocarbon polymeric composition having hydrophilic functional groups are disclosed as having connecting bridges or crosslinks having sulfonyl or carbonyl-containing groups joining polymeric chains, which can include loops joining portions of a polymeric chain.
U.S. Pat. No. 6,902,676, incorporated herein by reference in its entirety, discloses porous hydrophilic membranes wettable by water, the hydrophilic membrane comprising a porous inert support on which an amorphous ionomer is impregnated, the hydrophilic membranes being characterized in that they have a water permeability higher than 1 l/(h m2 atm), and in some cases higher than 500 l/(h m2 atm); the ionomer being under amorphous form or with crystallinity below 5% and having the hydrophilic group in the acid form. Bistretched PTFE base Goretex membrane having a porosity of 0.2 microns and thickness of 40 microns is used. Membranes having a high permeability contain an impregnated ionomer amount from 0.5 to 10% by weight (support+ionomer). When 20% by weight ionomer to 30% by weight ionomer is used membranes both partially and totally occluded to gases are found. This patent discloses that the (per)fluorinate ionomers can be crosslinked but that membranes obtainable by carrying out the crosslinking show a water permeability lower than the porous non-crosslinked ones, and this depends on the crosslinking entity. It is further disclosed that crosslinking allows an increase in the ionomer amount which coats the support walls. Two working examples in this patent describe the preparation of crosslinked porous membranes using fluorinated solvents with 16 wt % ionomer and 33 weight % ionomer of the total mass of the impregnated microporous membrane.
Benezra in U.S. Pat. No. 4,470,859 discloses a method for forming a hydrophilic coating upon a porous substrate such as a reticulate electrode or a filter from a dispersed, perfluorocarbon copolymer; the coating is not crosslinked. According to Benezra, the perfluorocarbon copolymer employed in such coatings should have an equivalent weight of not in excess of about 1500 so as to reasonably assure the presence of sufficient sulfonyl and/or carbonyl based or derived functional groups for providing hydrophilic properties to the porous or microporous substrate. Further, Benezra discloses where an abundance of functional groups are present per unit of copolymeric perfluorocarbon, the coating applied to a porous or microporous substrate may be excessively soluble in, for example, an aqueous fluid, or may be aggressively attacked by materials in contact with the coating. Benezra discloses that the equivalent weight of perfluorocarbon copolymer employed be not less than about 900 where pendent functionality of the copolymeric perfluorocarbon is carbonyl based or derived, and not less than about 950 where the pendent functionality is sulfonyl based or derived.
Benezra further discloses that the coating dispersion should be sufficiently viscous to be relatively readily retained within the microporous infrastructure while dispersion media is removed to leave a perfluorocarbon copolymeric coating upon substantially all surfaces of the infrastructure of a microporous substrate. According to Benezra, where, for viscosity or other reasons, it appears that dispersion or solution within the openly microporous substrate may flow out during dispersion media removal, utilization of tumbling techniques during removal of the dispersion media or solvent may assist in retaining perfluorocarbon copolymer within the infrastructure. Alternatively, Benezra discloses that deposition of the perfluorocarbon copolymer upon surfaces of the infrastructure of the microporous substrate can be enhanced by precipitating the copolymeric perfluorocarbon in situ within the infrastructure. Retention of such large amounts of coating on the membrane surfaces can lead to low water permeability, especially for microporous membranes with pore sizes of 0.2 microns or less.
U.S. Pat. No. 6,576,100 discloses crosslinked sulphonic fluorinated ionomers having an equivalent weight 380-1300 g/eq, comprising monomeric units deriving from one or more fluorinated monomers containing at least one ethylene unsaturation and from fluorinated monomeric units containing sulphonyl groups —SO2F in an amount such as to give the indicated equivalent weight. According to the disclosure, generally the larger the amount of sulphonic groups, ionomers having a lower equivalent weight, the better the efficiency of the ionomer in the application, both in terms of ion exchange capability in electrochemical applications, and in terms of the catalyst activity in catalysis applications. According to the disclosure, in electrochemical applications, for example in fuel cells, there is a direct correlation between the ionomer conductivity and the retention of water of the ionomer. According to this patent, the higher presence of ionic groups increased the ionic conductivity, and within certain limits, the amount of water that the polymer was able to keep.
Bacino in U.S. Pat. No. 7,306,729 discloses porous PTFE membranes that can be constructed as composite filters or composite vents, for example by layering the membrane with one or more additional layers that may provide support or protection to the membrane. The additional layer or layers may or may not be bonded to the membrane, depending on the end-use requirements. According to Bacino, these membranes can be rendered hydrophilic (water-wettable under little or no pressure) by various techniques making them usable in liquid filtration applications which involve, for example, filtration of aqueous fluids. According to the specification, a porous PTFE membrane was treated to render it hydrophilic by soaking in a solution of 1% polyvinyl alcohol (PVA) in a 50/50 mixture of isopropyl alcohol/deionized water. A PVA coating on a porous membrane would not be stable in oxidative, highly alkaline, and high temperature corrosive environments expected in cleaning baths used in semiconductor processing; a PVA coated porous membrane would therefore be expected to dewet and become hydrophobic during extended use in such baths.
Accordingly, there is a continuous need for microporous membranes with improved non-dewetting characteristics, that are wet with aqueous solutions containing reduced amounts of organic solvents, and that have good flow characteristics.